I can already predict that you will be confused by what I'm telling you. Let me try to explain it a little more clearly.

Let's say you have a morph. At 0 value none of the vertices are being influenced. Moving up to 1 the morph gradually appears, growing toward its determined form which will be reached at 1.

Moving beyond 1 the morph becomes stronger influencing the vertices often to an exaggeration of the it's natural determined form. Sometimes, with morphs that mimic scaling or were created with scaling this can work out well. Morphs aren't the same thing as scales though.

So what happens when you dial a morph backwards from one to 0? The morph is subtracted, gradually reverting to the objects default form.

The foreskin morphs you created aren't mimicking a scale, they're a very specific form you've created. If you created your foreskin forms around the non default morph shape that is occuring in Head Scale at 2, dialing it back to one isn't a following along reverse to perfect fit, it's a subtraction. Basically erasing the morph.

Now if you had created it at 1, the morph would grow to fit along to value 2. It's an increase in strength. Sometimes a fix is needed but usually the results are pretty close, depending. Especially if it's a fit to a morph that's mimicking scaling. Dialing it back to 1 would return it to its default morph shape, which is a perfect fit as you designed it, not a subtraction.

Make sense?